Nikon makes a good camera but keep in mind that the D40 won't auto focus with many older lenses so your compatibility with your wife's camera might be limited.
I own a Canon D60 and have been quite happy with it despite it's advanced age. the newest rebel is twice as good in almost every way for a quarter of what the D60 used to retail at.
The nice thing about having a DSLR is that you can keep the lenses when you switch bodies. Unlike electronics, nice glass doesn't lose much value over time with Canon lenses seeming to hold their value the best over time.